  We investigate the impact of geometric constriction on the viscous flow of
electron liquid through quantum point contacts. We provide analysis on the
electric potential distribution given the setup of a slit configuration and use
the method of conformal mapping to obtain analytical results. The potential
profile can be tested and contrasted experimentally with the scanning tunneling
potentiometry technique. We discuss intricate physics that underlies the Gurzhi
effect, i.e., the enhancement of conductivity in the viscous flow, and compare
results for different boundary conditions. In addition, we calculate the
temperature dependence of the momentum relaxation time as a result of impurity
assisted quasiballistic interference effects and discuss various correlational
corrections that lead to the violation of Matthiessen's rule in the
hydrodynamic regime. We caution that spatially inhomogeneous profiles of
current in the Gurzhi crossover between Ohmic and Stokes flows might also
appear in the nonhydrodynamic limit where nonlocality plays an important role.
This conclusion is corroborated by calculation of dispersive conductivity in
the weakly impure limit.
