  Superconducting proximity junctions based on topological insulators are
widely believed to harbor Majorana-like bound states. The latter serves as a
paradigm non-local topological quantum computation protocols. Nowadays, a
search for topological phases in different materials, perspective for a
realization of topological qubits, is one of the central efforts in quantum
physics. It is motivated, in particular, by recent observation of anomalous ac
Josephson effect, which being a signature of Majorana physics. Its
manifestations, such as a fractional Josephson frequency and the absence of the
first (or several odd in more rare cases), Shapiro steps, were reported for
different materials. Here we study Shapiro steps in Nb/Bi2Te2.3Se0.7/Nb
junctions, based on ultrasmall single crystals of a 3D topological insulator
synthesized by a physical vapor deposition (PVD) technique. We present evidence
that our junctions are ballistic. When subjected to microwave radiation, the
junctions exhibit Shapiro steps, but the first step is missing. Typically it is
assumed that the missing first step (MFS) effect cannot be observed in the
presence of quasiparticle poisoning due to suppression of the 4{\pi}-periodic
component. Our findings within the context of the RSJ-model of Josephson
junction dynamics show that such behaviour of samples corresponds to a specific
condition, requiring a minimum of 5% of the 4{\pi}-component for disappearance
of the first Shapiro step.
