  We provide detailed calculation of the a.c. conductivity in the case of
1/r-Coulomb interacting massless Dirac particles in graphene in the
collisionless limit when \omega >> T. The analysis of the electron self-energy,
current vertex function and polarization function, which enter into the
calculation of physical quantities including the a.c. conductivity, is carried
out by checking the Ward-Takahashi identities associated with the electrical
charge conservation and making sure that they are satisfied at each step. We
adopt a variant of the dimensional regularization of Veltman and t'Hooft by
taking the spatial dimension D=2-\epsilon, for \epsilon > 0. The procedure
adopted here yields a result for the conductivity correction which, while
explicitly preserving charge conservation laws, is nevertheless different from
the results reported previously in literature.
