  The current-dipole conductivity formula for doped three-dimensional Dirac
semimetals is derived by using a modified gauge-invariant tight-binding
approach. In a heavily doped regime, the effective number of charge carriers
$n_{\alpha \alpha}^{\rm eff}$ in the Drude contribution is found to be by a
factor of 4 larger than the nominal electron concentration $n$. However, its
structure is the same as in standard Fermi liquid theory. In a lightly doped
regime, on the other hand, the ratio $n_{\alpha \alpha}^{\rm eff}/n$ is much
larger, with much more complex structure of $n_{\alpha \alpha}^{\rm eff}$. It
is shown that the dc resistivity and reflectivity date measured in two TlBiSSe
samples can be easily understood, even in the relaxation-time approximation,
provided that finite quasiparticle lifetime effects in the momentum
distribution functions are properly taken into account.
