  We have measured the flux dependence of both real and imaginary conductance
of $GaAs/GaAlAs$ isolated mesoscopic rings at 310 MHz. The rings are coupled to
a highly sensitive electromagnetic superconducting micro-resonator and lead to
a perturbation of the resonance frequency and quality factor. This experiment
provides a new tool for the investigation of the conductance of mesoscopic
systems without any connection to invasive probes. It can be compared with
recent theoretical predictions emphasizing the differences between isolated and
connected geometries and the relation between ac conductance and persistent
currents. We observe $\Phi_0/2$ periodic oscillations on both components of the
magnetoconductance. The oscillations of the imaginary conductance whose sign
corresponds to diamagnetism in zero field, are 3 times larger than the Drude
conductance $G_0$. The real part of the periodic magnetoconductance is of the
order of $0.2 G_0$ and is apparently negative in low field. It is thus notably
different from the weak localisation oscillations observed in connected rings,
which are much smaller and opposite in sign.
