Low-Temperature Saturation of the Dephasing Time and Effects of Microwave Radiation on Open Quantum Dots

The dephasing time ${\ensuremath{\tau}}_{\ensuremath{\phi}}$ of electrons in open semiconductor quantum dots, measured using ballistic weak localization, is found to saturate below $\ensuremath{\sim}100\mathrm{mK}$, roughly twice the electron base temperature, independent of dot size. Microwave radiation deliberately coupled to the dots affects quantum interference indistinguishably from elevated temperatures, suggesting that direct dephasing due to radiation is not the cause of the observed saturation. Coulomb blockade measurements show that the applied microwaves create sufficient source-drain voltages to account for dephasing due to Joule heating.