Temperature dependence of the conductivity of ballistic graphene

We investigate the temperature dependence of 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 approximately square root(|n|) with negative corrections at small T due to the T dependence of the chemical potential. At moderate densities the conductivity is a nonmonotonic function of T and n, exhibiting a minimum at T=0.693 hv square root(|n|) where v is the Fermi velocity. We discuss two kinds of Fabry-Perot oscillations in short nanoribbons and their stability at finite temperatures.