  We combine electromechanical measurements with {\em ab initio} density
functional calculations to settle the controversy about the origin of
torsion-induced conductance oscillations in multi-wall carbon nanotubes.
According to our observations, the oscillation period is inversely proportional
to the squared diameter of the nanotube, as expected for a single-wall nanotube
of the same diameter. This is supported by our theoretical finding that
differential torsion effectively decouples the walls of a multi-wall nanotube
near the Fermi level and moves the Fermi momentum across quantization lines. We
exclude the alternative explanation linked to registry changes between the
walls, since it would cause a different diameter dependence of the oscillation
period.
