Phase-breaking rates from conductance fluctuations in a quantum dot

We report measurements of universal-like conductance fluctuations in a quasiballistic approximately 2 mu m2 GaAs/AlGaAs quantum dot with adjustable 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 conductance through the dot, consistent with a semiclassical picture of escape through the leads, and provides a means of estimating the phase 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.