Ballistic focusing mobility criterion

In a parabolic band, transport mobility and mean free path obey $\ell =\mu \hslash k_F/e$. The rough condition $\ell \gtrsim L$ gives $\mu \gtrsim eL/(\hslash k_F)$, 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.