According to random-matrix theory, interference effects in the conductance of a ballistic chaotic quantum dot should vanish $\propto(\tau_{ \phi}/\tau_{ D})^{ p}$ when the dephasing time $\tau_{ \phi}$ becomes small compared to the mean dwell time $\tau_{ D}$. Aleiner and Larkin have predicted that the power law crosses over to an exponential suppression $\propto\exp(-\tau_{ E}/\tau_{ \phi})$ when $\tau_{ \phi}$ drops below the Ehrenfest time $\tau_{ E}$. We report the first observation of this crossover in a computer simulation of universal conductance fluctuations. Their theory also predicts an exponential suppression $\propto\exp(-\tau_{ E}/\tau_{ D})$ in the absence of dephasing -- which is not observed. We show that the effective random-matrix theory proposed previously for quantum dots without dephasing explains both observations.
Categories: cond-mat.mes-hall