 Charged-current W branching-fraction counting

Neglect final-state masses and QCD corrections. Each of the three leptonic W boson decay channels contributes one common unit of width. Each open quark channel contributes $3|V_{ij}{|}^2$, with the factor three counting final colours. The top quark is kinematically inaccessible in an on-shell W decay because $m_t>m_W$. Thus $B(W\to \ell \nu )=1/D\simeq 1/9$ and $B(W^{-}\to b\bar{u},b\bar{c})=3(|V_{ub}{|}^2+|V_{cb}{|}^2)/D$. The large $V_{tb}$ does not open a forbidden top-quark channel.