  In order to investigate the effect of electronic phase coherence on screening
we have measured the flux dependent polarizability of isolated mesoscopic rings
at 350 MHz. At low temperature (below 100 mK) both non-dissipative and
dissipative parts of the polarizability exhibit flux oscillations with a period
of half a flux quantum in a ring. The sign and amplitude of the effect are in
good agreement with recent theoretical predictions. The observed positive
magneto-polarizability corresponds to an enhancement of screening when time
reversal symmetry is broken. The effect of electronic density and temperature
are also measured.
