= Heat-current relaxation time
{title2=$\tau_Q$}
A <heat-current relaxation time> measures the decay of a departure from equilibrium carrying heat. It need not equal the <momentum relaxation time> or the time between arbitrary scattering events. In an isotropic degenerate-electron estimate, $K_e=c_ev_F^2\tau_Q/3$. Small-angle inelastic <electron-phonon scattering> can remove an excitation energy of order $k_BT$ while barely changing the electron's velocity direction, thus relaxing heat more efficiently than charge flow.
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