Two-gap superconductivity in MgB$_{2}$: clean or dirty?
  A large number of experimental facts and theoretical arguments favor a two-gap model for superconductivity in MgB$_{2}$. However, this model predicts strong suppression of the critical temperature by interband impurity scattering and, presumably, a strong correlation between the critical temperature and the residual resistivity. No such correlation has been observed. We argue that this fact can be understood if the band disparity of the electronic structure is taken into account, not only in the superconducting state, but also in normal transport. 
I. I. Mazin, O. K. Andersen, O. Jepsen, O. V. Dolgov, J. Kortus, A. A.   Golubov, A. B. Kuz'menko, D. van der Marel