  We determine the characteristics of the 7mm to 20cm wavelength radio
variability in Sgr A* on time scales from days to three decades. The amplitude
of the intensity modulation is between 30 and 39% at all wavelengths. Analysis
of uniformly sampled data with proper accounting of the sampling errors
associated with the lightcurves shows that Sgr A* exhibits no 57- or 106-day
quasi-periodic oscillations, contrary to previous claims. The cause of the
variability is investigated by examining a number of plausible scintillation
models, enabling those variations which could be attributed to interstellar
scintillation to be isolated from those that must be intrinsic to the source.
Thin-screen scattering models do not account for the variability amplitude on
most time scales. However, models in which the scattering region is extended
out to a radius of 50-500pc from the Galactic Center account well for the broad
characteristics of the variability on >4-day time scales. The ~ 10% variability
on <4-day time scales at 0.7-3cm appears to be intrinsic to the source. The
degree of scintillation variability expected at millimeter wavelengths depends
sensitively on the intrinsic source size; the variations, if due to
scintillation, would require an intrinsic source size smaller than that
expected.
