  We study the temperature-dependent corrections to the conductance due to
electron-electron (e-e) interactions in clean two-dimensional conductors, such
as lightly doped graphene or other Dirac matter. We use semiclassical Boltzmann
kinetic theory to solve the problem of collision-dominated transport between
reflection-free contacts. Time-reversal symmetry and the kinematic constraints
of scattering in two dimensions (2D) ensure that inversion-odd and
inversion-even distortions of the quasiparticle distribution relax with
parametrically different rates at low temperature. This entails the surprising
result that at lowest temperatures the conductance of very long samples tends
to the noninteracting, ballistic conductance, despite the relaxation of the
quasiparticle distribution to a drifting equilibrium. The relative correction
to the conductance depends on the ratio of relaxation rates of even and odd
modes and scales as delta G/G_{ballistic}~(T/E_F)[Log(E_F/T)]^{1/2}, in stark
contrast to the behavior in other dimensionalities. This holds generally in 2D
systems with simply connected and convex but otherwise arbitrary Fermi
surfaces, as long as e-e scattering processes are dominant and umklapp
scattering is negligible. These results are especially relevant to the bulk of
wide and long suspended high-mobility graphene sheets.
