  We report measurements of Aharonov-Bohm oscillations in normal metal rings in
the presence of high frequency electromagnetic fields. The power dependence of
the decoherence time scale tau_phi(P) agrees well with the anticipated power
law tau_phi proportional to P^(-1/5) when the field-induced decoherence rate
tau_ac^(-1) is large compared to the intrinsic decoherence rate tau_o^(-1),
measured in the absence of external fields. As theoretically expected, we
observe a decline in field-induced decoherence when tau_ac^(-1)<=tau_o^(-1).
The frequency dependence of tau_phi shows a minimum in the oscillation
amplitude at a characteristic frequency, omega_(ac)=1/tau_(o), where tau_(o) is
evaluated from the oscillation amplitude using the standard mesoscopic theory.
Both the suppression in the oscillation amplitude and the concomitant change in
conductivity allow a direct measurement of the intrinsic decoherence time
scale.
