Nonequilibrium dephasing in two-dimensional indium oxide films

We report on results of resistance R and magnetoresistance in diffusive indium oxide films measured down to T=0.28 K. Analyzing the data using weak-localization theory shows that the phase-coherent time tau (phi) increases without bound as T-->0. However, this result is obtained only when the voltage applied to the sample V is sufficiently small. When V is not small, tau (phi) may appear to "saturate" while R continues to increase as T-->0. Possible reasons for this intriguing behavior are discussed. It is argued that in out-of-equilibrium situations R(T) and tau (phi)(T) need not behave similarly. We suggest a heuristic picture, involving two-level systems, which might be consistent with our observations.