  Electron hydrodynamics typically emerges in electron fluids with a high
electron-electron collision rate. However, new experiments with thin flakes of
WTe$_2$ have revealed that other momentum-conserving scattering processes can
replace the role of the electron-electron interaction, thereby leading to a
novel, so-called para-hydrodynamic regime. Here, we develop the kinetic theory
for para-hydrodynamic transport. To this end, we consider a ballistic electron
gas in a thin 3-dimensional sheet where the momentum-relaxing ($\ell_{mr}$) and
momentum-conserving ($\ell_{mc}$) mean free paths are decreased due to boundary
scattering from a rough surface. The resulting effective mean free path of the
electronic flow is then expressed in terms of microscopic parameters of the
sheet boundaries, predicting that a para-hydrodynamic regime with $\ell_{mr}\gg
\ell_{mc}$ emerges generically in ultraclean three-dimensional materials. Using
our approach, we recover the transport properties of WTe$_2$ in the
para-hydrodynamic regime in good agreement with existing experiments.
