  Quantum transport in three-dimensional massless Dirac electron system with
long-range Gaussian impurities is studied theoretically using a self-consistent
Born approximation (SCBA). We find that the conductivity significantly changes
the behavior at a certain scattering strength which separates the weak and
strong disorder regimes. In the weak disorder regime, the SCBA conductivity
mostly agrees with the Boltzmann conductivity, while the agreement fails near
the Dirac point where the SCBA conductivity drops to zero linearly to the Fermi
energy. In the strong disorder regime, the conductivity is smooth and finite
near the Dirac point, and the minimum conductivity becomes larger in increasing
the disorder potential, contrary to the usual metallic behavior. The theory
applies to three dimensional gapless band structures, including Weyl
semimetals.
