  We report on several low temperature experiments supporting the presence of
Majorana fermions in superconducting lead nanowires fabricated with a scanning
tunneling microscope. These nanowires are the connecting bridges between the
STM tip and the sample resulting from indentation processes. We show here that
by a controlled tuning of the geometry of the nanowire region, in which
superconductivity is confined by applied magnetic fields, the conductance
curves obtained in these situations are indicative of topological
superconductivity and Majorana fermions. The most prominent feature of this
behavior is the emergence of a zero bias peak in the conductance curves,
superimposed on a background characteristic of the conductance between a normal
metal and a superconductor in the Andreev regime. The zero bias peak emerges in
some nanowires when a magnetic field larger that the lead bulk critical field
is applied. This field drives one of the electrodes into the normal state while
the other, the tip, remains superconducting on its apex. Meanwhile a
topological superconducting state appears in the connecting nanowire of
nanometric size.
