  We demonstrate that the radiation induced "zero-resistance state" observed in
a two-dimensional electron gas is a result of the non-trivial structure of the
density of states of the systems and the photon assisted transport. A toy model
of a structureless quantum tunneling junction where the system has oscillatory
density of states catches most of the important features of the experiments. We
present a generalized Kubo-Greenwood conductivity formula for the photon
assisted transport in a general system, and show essentially the same nature of
the transport anomaly in a uniform system.
