I am following Sakurai's Modern Quantum Mechanics, 3ed.
Define the scattering, or S-matrix elements as $$S_{ni} \equiv \delta_{ni} - 2\pi i\delta(E_n - E_i)T_{ni}.$$ We can then derive the differential scattering cross section $$\frac{d\sigma}{d\Omega} = \left(\frac{mL^3}{2\pi\hbar^2}\right)^2 \lvert T_{ni}\lvert^2.$$ How do I interpret the differential scattering cross section purely in quantum mechanical terms? For instance, "...the differential scattering cross section literally is defined to be the time derivative of the probability amplitude corresponding from a transition from an initial state to a final state per probability current per solid angle, which physically means..."