Phase breaking as a probe of the intrinsic level spectrum of open quantum dots

We measure the dependence of the phase breaking time on temperature and on dc voltage in ballistic split-gate quantum dots, the lead openings of which are defined by means of quantum point contacts. Estimates for the phase breaking time are obtained by studying the high-field evolution of the reproducible fluctuations, observed in the low-temperature magneto-conductance of the devices, and from the quenching of quantum interference induced by the dc bias. Varying either temperature or dc bias, a saturation of the phase breaking time is observed for sufficiently low values of these parameters. The energy scale at which the saturation onsets is found to be comparable to the average spacing of discrete levels within the quantum dot, indicative of a crossover to zero-dimensional phase breaking. Based on these observations, we suggest that studies of electron phase coherence may provide a powerful probe of the intrinsic eigenspectra of open quantum dots.