  We study the $B^0_s-\bar{B^0_s}$ mixing amplitude in Standard Model by
computing the relevant hadronic matrix element in the static limit of lattice
HQET with the Neuberger light quark action. In the quenched approximation, and
after matching to the $\bar{\rm MS}$ scheme in QCD, we obtain $B^{\bar{\rm
MS}}_{B_s}(m_b)=0.940(16)(22)$.
