  Within a Kubo formalism, we study dc transport and ac optical properties of
3D Dirac and Weyl semimetals. Emphasis is placed on the approach to charge
neutrality and on the differences between Dirac and Weyl materials. At charge
neutrality, the zero-temperature limit of the dc conductivity is not universal
and also depends on the residual scattering model employed. However, the Lorenz
number L retains its usual value L_0. With increasing temperature, the
Wiedemann-Franz law is violated. At high temperatures, L exhibits a new plateau
at a value dependent on the details of the scattering rate. Such details can
also appear in the optical conductivity, both in the Drude response and
interband background. In the clean limit, the interband background is linear in
photon energy and always extrapolates to the origin. This background can be
shifted to the right through the introduction of a massless gap. In this case,
the extrapolation can cut the axis at a finite photon energy as is observed in
some experiments. It is also of interest to differentiate between the two types
of Weyl semimetals: those with broken time-reversal symmetry and those with
broken spatial-inversion symmetry. We show that, while the former will follow
the same behaviour as the 3D Dirac semimetals, for the zero magnetic field
properties discussed here, the latter type will show a double step in the
optical conductivity at finite doping and a single absorption edge at charge
neutrality. The Drude conductivity is always finite in this case, even at
charge neutrality.
