  We have measured the electromagnetic response of micron-size isolated
mesoscopic GaAs/GaAlAs square dots down to temperature T=16mK, by coupling them
to an electromagnetic micro-resonator. Both dissipative and non dissipative
responses exhibit a large magnetic field dependent quantum correction, with a
characteristic flux scale which corresponds to a flux quantum in a dot. The
real (dissipative) magnetoconductance changes sign as a function of frequency
for low enough density of electrons. The signal observed at frequency below the
mean level spacing corresponds to a negative magnetoconductance, which is
opposite to the weak localization seen in connected systems, and becomes
positive at higher frequency. We propose an interpretation of this phenomenon
in relation to fundamental properties of energy level spacing statistics in the
dots.
