  ZrSiS exhibits a frequency-independent interband conductivity $\sigma(\omega)
= \rm{const}(\omega) \equiv \sigma_{\rm{flat}}$ in a broad range from 250 to
2500 cm$^{-1}$ (30 - 300 meV). This makes ZrSiS similar to
(quasi)two-dimensional Dirac electron systems, such as graphite and graphene.
We assign the flat optical conductivity to the transitions between
quasi-two-dimensional Dirac bands near the Fermi level. In contrast to
graphene, $\sigma_{\rm{flat}}$ is not supposed to be universal but related to
the length of the nodal line in the reciprocal space, $k_{0}$. When
$\sigma_{\rm{flat}}$ and $k_{0}$ are connected by a simple model, we find good
agreement between experiment and theory. Due to the spin-orbit coupling, the
discussed Dirac bands in ZrSiS possess a small gap $\Delta$, for which we
determine an upper bound max($\Delta$) = 30 meV from our optical measurements.
At low temperatures the momentum-relaxation rate collapses, and the
characteristic length scale of momentum relaxation is of the order of microns
below 50 K.
