  We present a model which provides a plausible explanation of the effect of
zero-resistance and zero-conductance states in two-dimensional electron systems
subjected to a magnetic field and irradiated with microwaves observed in a
number of experiments and of the effect main features. The model is based on
the concept of absolute negative conductivity associated with photon-assisted
scattering of electrons on impurities. It is shown that the main features of
the effect can be attributed to the interplay of different electron scattering
mechanisms.
