  We present new 5.2-14.5 micron low-resolution spectra of 14 mid-L to mid-T
dwarfs, as well as 3.0-4.1 micron spectra for five of these dwarfs. These data
are supplemented by existing red and near-infrared spectra (0.6-2.5 micron), as
well as red through mid-infrared spectroscopy of seven other L and T dwarfs
presented by Cushing et al. (2008). We compare these spectra to those generated
from the model atmospheres of Saumon & Marley (2008). The models reproduce the
observed spectra well, except in the case of one very red L3.5 dwarf, 2MASS
J22244381-0158521. The broad wavelength coverage allows us to constrain almost
independently the four parameters used to describe these photospheres in our
models: effective temperature (Teff), surface gravity, grain sedimentation
efficiency (fsed) and vertical gas transport efficiency (Kzz).
  The sample of L3.5 to T5.5 dwarfs spans the range 1800 < Teff < 1000 K, with
an L-T transition (spectral types L7-T4) that lies between 1400 K and 1100K for
dwarfs with typical near-infrared colors; bluer and redder dwarfs can be 100 K
warmer or cooler, respectively, when using infrared spectral types. In this
model analysis, fsed increases rapidly between types T0 and T4, indicating that
increased sedimentation can explain the rapid disappearance of clouds at this
stage of brown dwarf evolution. There is a suggestion that the transition to
dust-free atmospheres happens at lower temperatures for lower gravity dwarfs.
