  The effect of magnetic impurities on the ballistic conductance of
nanocontacts is, as suggested in recent work, amenable to ab initio study
\cite{naturemat}. Our method proceeds via a conventional density functional
calculation of spin and symmetry dependent electron scattering phase shifts,
followed by the subsequent numerical renormalization group solution of Anderson
models -- whose ingredients and parameters are chosen so as to reproduce these
phase shifts. We apply this method to investigate the Kondo zero bias anomalies
that would be caused in the ballistic conductance of perfect metallic (4,4) and
(8,8) single wall carbon nanotubes, ideally connected to leads at the two ends,
by externally adsorbed Co and Fe adatoms. The different spin and electronic
structure of these impurities are predicted to lead to a variety of Kondo
temperatures, generally well below 10 K, and to interference between channels
leading to Fano-like conductance minima at zero bias.
