  The structure of AGN accretion disks on sub-parsec scales can be probed
through free-free absorption of synchrotron emission from the base of symmetric
radio jets. We present new VLBA observations of the nearby FR-I radio galaxy
NGC 4261 at 22 and 43 GHz, and combine these with previous VLBA observations at
1.6 and 8.4 GHz to map absorption caused by an inner accretion disk. Assuming
the disk is geometrically and optically thin and composed of a uniform 10^4 K
plasma, the average electron density in the inner 0.1 pc is 10^3 - 10^8 cm^-3.
Equating thermal gas pressure and magnetic field strength gives a disk magnetic
field of 10^-4 - 10^-2 Gauss at 0.1 pc. The jet opening angle is between 0.3
and 20 degrees during the first 0.2 pc of the jet, and must be less than 5
degrees during the first 0.8 pc. We include an appendix containing expressions
for a simple, optically thin, gas pressure dominated accretion disk model which
may be applicable to other galaxies in addition to NGC 4261.
