  Umklapp scattering determines the conductivity of clean metals. In typical
quasi one-dimensional Fermi liquids with an open Fermi surface, certain
pseudo-momenta do not decay by 2-particle collisions even in situations where
Umklapp scattering relaxes the momentum of the quasi particles efficiently. Due
to this approximate conservation of pseudo-momentum, a certain fraction of the
electrical current decays very slowly and a well-pronounced low-frequency peak
emerges in the optical conductivity. We develop simple criteria to determine
under what conditions approximate pseudo-momentum conservation is relevant and
calculate within in Fermi liquid theory the weights of the corresponding
low-frequency peaks and the temperature dependence of the various relevant
decay rates. Based on these considerations, we obtain a qualitative picture of
the frequency and temperature dependence of the optical conductivity of an
anisotropic Fermi liquid.
