  We calculate the electronic transport properties of a system which is
irradiated by a homogeneous microwave field. Within a Boltzmann equation
approach, a general expression for the conductivity tensor is derived and
evaluated for a quasi one-dimensional ballistic quantum wire and a two
dimensional system with impurity scattering. For the latter, deviations from
the Drude result are predicted for the conductivity tensor. This should be
observable in systems where the scattering rate depends noticeably on the Fermi
energy. The deviations are calculated explicitly for the case of a 2d Silicon
layer, where they strongly depend on the microwave polarization and the Fermi
energy.
