  The cross section for the reaction $e^{-}e^{-} \to \ell^-\ell^-$
($\ell=\mu,\tau$) is calculated in models with heavy Majorana neutrinos
mediating lepton number violating amplitudes at the loop level. The
contributing four-point functions are evaluated exactly (numerically) taking
into account the full propagator dependence on external momenta, thereby
extending %an earlier approximate low energy calculation to the energy range of
interest for the next linear colliders an earlier approximate low energy
calculation. The amplitude shows a non-decoupling behaviour relative to the
heavy Majorana neutrino masses, but due to the stringent bounds on heavy-light
mixing the signal cross section attains observable values only for the less
constrained $\tau$ signal. The cross section induced by lepton number violation
in the $SU(2)_L$ doublet sneutrino sector of supersymmetric extensions of the
standard model is constrained by the upper limits on neutrino masses and
probably too tiny to be observable.
