Transport mean free path (${\ell }_{\mathrm{tr}}=v_F{\tau }_{\mathrm{tr}}=\mu \hslash k_F/e$)

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.