  The paper discusses the master equation approach to derivation of the
Esaki-Tsu equation for drift current. It is shown that the relaxation term in
the master equation can be identified by measuring the velocity distribution of
the carriers. We also show that the standard form of the relaxation term, used
earlier to derive Esaki-Tsu equation, predicts unphysical velocity distribution
and suggest a more elaborated relaxation term, which is argued to correctly
capture the effect of bosonic bath in experiments on atomic current in optical
lattices.
