Transport mean free path
= Transport mean free path
{title2=$\ell_{\rm tr}=v_F\tau_{\rm tr}=\mu\hbar k_F/e$}
The length associated with relaxation of carrier momentum. In a parabolic band, <electron mobility> measures the <momentum relaxation time>, giving the displayed expression. Frequent weak small-angle scattering can make the single-particle scattering time shorter than the transport time, and <electron phase coherence> has its own lifetime.