We solve the Boltzmann equation for electrons moving in a two-dimensional
plane of square symmetry in the presence of a transverse magnetic field. We
assume that there are two sources of scattering: a large momentum-independent
scattering on a collective mode of the electron system, and a smaller
momentum-dependent forward scattering on impurities. We show that the effect of
impurities on the longitudinal and Hall conductivities is of the same order of
magnitude