Ballistic focusing mobility criterion
= Ballistic focusing mobility criterion
In a parabolic band, transport mobility and <mean free path> obey $\ell=\mu\hbar k_F/e$. The rough condition $\ell\gtrsim L$ gives $\mu\gtrsim eL/(\hbar k_F)$, whereas surviving a first semicircle uses path length $\pi L/2$. Clear peaks usually need a larger margin and a specular boundary, so this is an observability scale rather than a universal sharp threshold.