  We investigate the temperature dependence of the conductivity in ballistic
graphene using Landauer transport theory. We obtain results which are
qualitatively in agreement with many features recently observed in transport
measurements on high mobility suspended graphene. The conductivity \sigma at
high temperature T and low density n grows linearly with T, while at high n we
find \sigma ~ |n|^1/2 with negative corrections at small T due to the
T-dependence of the chemical potential. At moderate densities the conductivity
is a non-monotonic function of T and n, exhibiting a minimum at T=0.693 \hbar v
|n|^1/2 where v is the Fermi velocity. We discuss two kinds of Fabry-Perot
oscillations in short nanoribbons and their stability at finite temperatures.
