  We present polarization observations of two Galactic plane fields centered on
Galactic coordinates (l,b)=(0 deg,0 deg) and (329 deg, 0 deg) at Q- (43 GHz)
and W-band (95 GHz), covering between 301 and 539 square degrees depending on
frequency and field. These measurements were made with the QUIET instrument
between 2008 October and 2010 December, and include a total of 1263 hours of
observations. The resulting maps represent the deepest large-area Galactic
polarization observations published to date at the relevant frequencies with
instrumental rms noise varying between 1.8 and 2.8 uK deg, 2.3-6 times deeper
than corresponding WMAP and Planck maps. The angular resolution is 27.3' and
12.8' FWHM at Q- and W-band, respectively. We find excellent agreement between
the QUIET and WMAP maps over the entire fields, and no compelling evidence for
significant residual instrumental systematic errors in either experiment,
whereas the Planck 44 GHz map deviates from these in a manner consistent with
reported systematic uncertainties for this channel. We combine QUIET and WMAP
data to compute inverse-variance-weighted average maps, effectively retaining
small angular scales from QUIET and large angular scales from WMAP. From these
combined maps, we derive constraints on several important astrophysical
quantities, including a robust detection of polarized synchrotron spectral
index steepening of ~0.2 off the plane, as well as the Faraday rotation measure
toward the Galactic center (RM=-4000 +/- 200 rad m^-2), all of which are
consistent with previously published results. Both the raw QUIET and the
co-added QUIET+WMAP maps are made publicly available together with all
necessary ancillary information.
