Validity of kinetic theory I am reading this paper, which uses (chiral) kinetic theory. 
The authors write:

The Boltzmann method is valid only in the semiclassical limit, where $\omega_B \tau \ll 1$, $\omega_B \ll \mu$ and $\mu\tau\gg 1$. Here, $\omega_B$ is the magnetic frequency, $v_F$ is the Fermi velocity, and $\tau$ is the quasiparticle lifetime. (In this paper, we set $\hbar = 1$ and the energy of WP as 0.) In the semiclassical limit, the level smearing caused by the finite quasiparticle lifetime is much larger than the Landau-level splitting, but it is much smaller than the chemical potential; hence, the Landau-level quantization can be ignored, and the Fermi surface remains well defined. Therefore, in the semiclassical limit, the collective dynamics of a Fermi-liquid system can be described by the quasiparticle distribution function $n({\bf k},{\bf r},t)$ through the following Boltzmann equation

$$
\big[\partial_t + {\bf \dot{r}}\cdot\nabla_{\bf r} - {\bf \dot{k}}\cdot\nabla_{\bf k}\big]\delta n ({\bf k}, {\bf r},t) = S[\delta n({\bf k}, {\bf r},t)],
$$
where $S[\delta n({\bf k}, {\bf r},t)]$ is a collision integral and $\delta n({\bf k},{\bf r},t)$ the variation in the distribution function.
The issue of validity of kinetic theory has been on my mind for a while. According to the authors of the text above, the chemical potential needs to be the biggest scale in the problem in order to use the (semiclassical) Boltzmann equation. I do not completely understand the explanation they give. Could somebody elaborate on the range of validity of the (semiclassical) Boltzmann equation?
Furthermore, what happens when we consider the limit $\mu \to 0$? In the case of a gapless system like a Dirac or Weyl semimetal, can we still use chiral kinetic theory? Does it break down and how?
 A: Kinetic theory is generically valid when the lifetime of the quasiparticle is long compared to the relevant scales in your problem; it is a theory where you assume quasiparticles are well-defined.
The statement about magnetic frequency is a statement about the importance of Landau level quantization, which they want to ignore; they want to treat the magnetic field "semiclassically."
The statement about chemical potential being "large" is to argue that they have a well-defined electron-like or hole-like Fermi surface, ie they are sitting well away from the Weyl point. Strictly speaking, this is not explicitly related to the regime of validity of kinetic theory. 
However, when the Fermi surface shrinks to a point, strong renormalization effects occur, enhancing scattering and potentially significantly reducing the lifetime of your quasiparticles. This would invalidate the kinetic theory picture. Another way of saying this is that in "normal" Fermi liquid (kinetic theory) physics, all the action happens at the Fermi surface. However, when the notion of Fermi surface breaks down, so does Fermi liquid (kinetic) theory. 
