  Starting from the quasiclassical equations for non-equilibrium Green's
functions we derive a simple kinetic equation that governs ac Josephson effect
in a superconducting quantum point contact at small bias voltages. In contrast
to existing approaches the kinetic equation is valid for voltages with
arbitrary time dependence. We use this equation to calculate
frequency-dependent linear conductance, and dc $I\!-\!V$ characteristics with
and without microwave radiation for resistively shunted quantum point contacts.
A novel feature of the $I\!-\!V$ characteristics is the excess current
$2I_c/\pi$ appearing at small voltages. An important by-product of our
derivation is the analytical proof that the microscopic expression for the
current coincides at arbitrary voltages with the expression that follows from
the Bogolyubov-de Gennes equations, if one uses appropriate amplitudes of
Andreev reflection which contain information about microscopic structure of the
superconductors.
