  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.

Mesoscale and Nanoscale Physics