Possible mechanism of the fractional-conductance quantization in a one-dimensional constriction

As is well known, there may arise situations when an interaction between electrons is attractive. A weak attraction should manifest itself strongly in one-dimensional (1D) systems, since it can create two-electron bound states. This paper interprets the 0.7 ${(2e}^2/h)$ conductance structure, observed recently in a one-dimensional constriction, as a manifestation of two-electron bound states formed in a barrier saddle point. The value 0.75 ${(2e}^2/h)$ follows naturally from the 3:1 triplet-singlet statistical weight ratio for the two-electron bound states, if the triplet energy is lower. Furthermore, the value 0.75 has to be multiplied by the probability T of the bound state formation during adiabatic transmission of two electrons into the 1D channel $\left(T\simeq 1\right).$ If the binding energy is larger than the subband energy spacing, the 0.7 structure can be seen even when the integer steps are smeared away by the temperature. Bound states of several electrons, if they exist, may give different steps at 1/2, 5/16, 3/16 etc., in the conductance. The latter results are sensitive to the length of the 1D system and the electron density at the barrier.