  We study theoretically the current-voltage characteristics of intrinsic
Josephson junctions in high-$T_c$ superconductors. An oscillation of the
breakpoint current on the outermost branch as a function of coupling $\alpha$
and dissipation $\beta$ parameters is found. We explain this oscillation as a
result of the creation of longitudinal plasma waves at the breakpoint with
different wave numbers. We demonstrate the commensurability effect and predict
a group behavior of the current-voltage characteristics for the stacks with a
different number of junctions. A method to determine the wave number of
longitudinal plasma waves from $\alpha$- and $\beta$-dependence of the
breakpoint current is suggested. We model the $\alpha$- and $\beta$-dependence
of the breakpoint current and obtain good agreement with the results of
simulation.
