  We present a broadband power spectral density function (PSD) measured from
extensive RXTE monitoring data of the low-luminosity AGN NGC 4258, which has an
accurate, maser-determined black hole mass of 3.9+/-0.1 * 10^7 solar masses. We
constrain the PSD break timescale to be greater than 4.5 d at >90% confidence,
which appears to rule out the possibility that NGC 4258 is an analogue of black
hole X-ray binaries (BHXRBs) in the high/soft state. In this sense, the PSD of
NGC 4258 is different to those of some more-luminous Seyferts, which appear
similar to the PSDs of high/soft state X-ray binaries. This result supports
previous analogies between LLAGN and X-ray binaries in the low/hard state based
on spectral energy distributions, indicating that the AGN/BHXRB analogy is
valid across a broad range of accretion rates.
