  We present results from the analysis of X-ray power density spectra and
coherence when GRS 1915+105 is in soft states. We use three data sets that
belong to $\mu$, $\phi$ and $\delta$ classes in Belloni et al (2000). We find
that the power density spectra appear to be complex, with several features
between 0.01 and 10 Hz. The coherence deviates from unity above a
characteristic frequency. We discuss our results in different models. The
corona size in the sphere-disk model implied by this break frequency is on the
order of 10$^4$ GM/c$^2$, which is unphysical. Our results are more consistent
with the prediction of the model of a planar corona sustained by magnetic
flares, in which the characteristic frequency is associated with the longest
time-scale of an individual flare, which is about eight seconds.
