Electroweak Corrections to the Top Quark Decay
  We have calculated the one-loop electroweak corrections to the decay t-> bW+, including the counterterm for the CKM matrix elements V(tb). Previous calculations used an incorrect delta V(tb) that led to a gauge dependent amplitude. However, since the contribution stemming from delta V(tb) is small, those calculations only underestimate the width by roughly one part in 10^5. 
S. M. Oliveira, L. Bruecher, R. Santos, A. Barroso