  We calculate the conductance of a circular constriction of radius $a$ in an
insulating diaphragm which separates two conducting half-spaces characterized
by the mean free path $\ell$. Our exact result interpolates between the Maxwell
conductance in diffusive ($\ell \ll a$) and the Sharvin conductance in
ballistic ($\ell \gg a$) transport regime. Following the earlier approach of
Wexler we find the explicit form of the Green's function for the linearized
Boltzmann operator. The formula for the conductance deviates by less than 11%
from the naive interpolation formula obtained by adding resistances in the
diffusive and the ballistic regime.
