  A three-level system with partially broken SU(3) symmetry immersed in a
metal, comprised of a unique non-interacting ground state and two-fold
degenerate excited states, exhibits a stable two-channel Kondo fixed point
within a wide range of parameters, as has been shown in previous work. Such
systems can, for instance, be realized by protons dissolved in a metal and
bound in the interstitial space of the host lattice, where the degeneracy of
excited rotational states is guaranteed by the space inversion symmetry of the
lattice. We analyze the robustness of the 2CK fixed point with respect to a
level splitting of the excited states and discuss how this may explain the
behavior of the well-known dI/dV spectra measured by Ralph and Buhrman on
ultrasmall quantum point contacts in a magnetic field.
