  It is shown that the ideal boundary between a perfectly conducting electrode
and electron liquid state acts as a contact whose conductance per unit area is
higher than the fundamental Sharvin conductance by a numerical coefficient $2
\alpha$, where $\alpha$ is slightly smaller than unity and depends on the
dimensionality of the system. If the boundary has a finite curvature, an
additional correction to the boundary conductance appears, which is
parametrically small as a product of the curvature by the electron-electron
mean free path length. The relation of the normal current density to the
voltage between the electrode and electron liquid represents itself a
hydrodynamic boundary condition for current-penetrable boundary. Calculations
of the conductance and potential distribution in microstructures by means of
numerical solution of the Boltzmann equation show that the concept of boundary
conductance works very good when the hydrodynamic transport regime is reached.
The superballistic transport, when the device conductance is higher than the
Sharvin conductance, can be realized in Corbino disk devices not only in the
hydrodynamic regime, although requires that the electron-electron scattering
must be stronger than the momentum-relaxing scattering. The theoretical results
for Corbino disks are consistent with recent experimental findings.
