# What is the role of $\hbar$ in quantum mechanics? [duplicate]

Planck's constant $$\hbar$$ appears in the Schrodinger equation:

$$i\hbar \frac{d|\psi\rangle}{dt}\ = \hat{H}|\psi\rangle$$

which implies for stationary states,

$$|\psi(x,t)\rangle=e^{-iE_n/\hbar}|\psi(x,t=0)\rangle \, .$$

What would you say is the role of $$\hbar$$ or is it worth considering the other formulations of quantum mechanics, like the Heisenberg equation of motion (where the operators carry the time evolution:

$$i\hbar \frac{d\hat{A}(t)}{dt} = [\hat{A}(t),H] + i\hbar\frac{\partial\hat{A}(t)}{\partial t} \, ?$$

Or is it worth considering this from the standpoint of the Path Integral Formulation where $$\hbar$$ is used to work out the probability amplitude of a specific path:

$$e^{iS/\hbar}.$$

I understand that $$\hbar$$ is often used to make equations dimensionally consistent and that it is often set to 1 when dealing with complicated equations. It is also sometimes stated that $$\hbar \to 0$$ is the "classical limit". I do not understand this.