  In some theoretical analyses of microwave-induced magnetoresistance
oscillations in high-mobility two-dimensional systems, the "inelastic
relaxation time" $\tau_{in}$ due to electron-electron scattering is evaluated
using an equilibrium distribution function $f^0$ in the absence of radiation,
and it is concluded that $\tau_{in}$ is much larger than $\tau_{q}$, the
single-particle relaxation time due to impurity scattering. However, under the
irradiation of a microwave capable of producing magnetoresistance oscillation,
the distribution function of the high-mobility electron gas deviates remarkably
from $f^0$ at low temperatures. Estimating $\tau_{in}$ using an approximate
nonequilibrium distribution function rather than using $f^0$, one will find the
system to be in the opposite limit $1/\tau_{in}\ll 1/\tau_{q}$ even for T=0 K.
Therefore, models which depend on the assumption $1/\tau_{in}\gg 1/\tau_{q}$
may not be justifiable.
