VLBA Continuum Observations of NGC 4258: Constraints on an   Advection-Dominated Accretion Flow
  We report a 3 sigma upper limit of 220 microJanskys on any 22-GHz continuum emission coincident with the central engine in NGC 4258. If NGC 4258 is powered by an advection-dominated accretion flow, this radio upper limit implies that the inner advection-dominated flow cannot extend significantly beyond about 100 Schwarzschild radii. 
J. R. Herrnstein, L. J. Greenhill, J. M. Moran, P. J. Diamond, M.   Inoue, N. Nakai, and M. Miyoshi