On the Status of $V_{td}$, $V_{ts}$ and $V_{tb}$
  The CKM matrix elements $V_{td}$, $V_{ts}$ and $V_{tb}$, while notoriously hard to measure directly in experiment, can be determined to great precision within the Standard Model (SM) with a minimal set of observables due to unitarity of $V_{\rm CKM}$. We discuss the potential modifications of these elements in extensions with non-unitary quark mixing matrices. 
J. Rohrwild
hep-ph