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The cross section for the reaction $e^{-}e^{-} \to \ell^-\ell^-$($\ell=\mu,\tau$) is calculated in models with heavy Majorana neutrinosmediating lepton number violating amplitudes at the loop level. Thecontributing four-point functions are evaluated exactly (numerically) takinginto account the full propagator dependence on external momenta, therebyextending %an earlier approximate low energy calculation to the energy range ofinterest for the next linear colliders an earlier approximate low energycalculation. The amplitude shows a non-decoupling behaviour relative to theheavy Majorana neutrino masses, but due to the stringent bounds on heavy-lightmixing the signal cross section attains observable values only for the lessconstrained $\tau$ signal. The cross section induced by lepton number violationin the $SU(2)_L$ doublet sneutrino sector of supersymmetric extensions of thestandard model is constrained by the upper limits on neutrino masses andprobably too tiny to be observable.Comment: Revtex 4, 5 figure

On the heavy Majorana neutrino and light sneutrino contribution to $e^-e^-\to\ell^- \ell^-$ , ( $\ell = \mu,\tau$ )