We discuss the potential of e(-)e(-) collisions at energies comparable to t
he masses of heavy Majorana neutrinos to reveal their mass and mixing param
eters. This potential is compared with the low-energy environment in neutri
noless double beta decay of complex nuclei. We give estimates for the short
-range repulsion between any pair of valence quarks bound inside two differ
ent nucleons giving rise to suppression factors in the range of 50-100 in a
mplitude for decays mediated by heavy flavors.