Sign Reversals of ac Magnetoconductance in Isolated Quantum Dots

We have measured the electromagnetic response of micron-size isolated GaAs/GaAlAs square dots down to $16\mathrm{mK}$ by coupling them to a microresonator. Both dissipative and reactive responses exhibit a large magnetic field dependent quantum correction, with a characteristic flux scale corresponding to a flux quantum in a dot. The dissipative magnetoconductance changes sign as a function of frequency for a low density of electrons. The signal observed at a frequency below the mean level spacing corresponds to a negative magnetoconductance, in contrast to weak localization in connected systems, and becomes positive at higher frequencies. We interpret this phenomenon in relation to the energy spectrum statistics in the dots.