PHASE-BREAKING RATES FROM CONDUCTANCE FLUCTUATIONS IN A QUANTUM-DOT

We report measurements of universal-like conductance fluctuations in a quasiballistic approximately 2 mum2 GaAs/AlGaAs quantum dot with adju stable point contact leads. Measurements cover a range of conductance G from nearly isolated, [G] approximately 0.1e2/h, to several modes in each lead, [G] approximately 5e2/h. The characteristic magnetic field scale of the fluctuations is found to increase with increasing mean c onductance through the dot, consistent with a semiclassical picture of escape through the leads, and provides a means of estimating the phas e breaking time tau(phi) for electrons inside the dot. The fluctuation amplitude is found to increase with increasing dot conductance, then begins to saturate once a few conducting channels are open.