  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.
