Limiting the Abundance of LIGO/Virgo Black Holes with Microlensing
  Observations of Quasars of Finite Size
  We present a simple but general argument that strongly limits the abundance
of Primordial Black Holes (PBHs) (or other unknown population of compact
objects) with masses similar to those determined by LIGO/Virgo from BH binary
mergers. We show that quasar microlensing can be very sensitive to the mass of
the lenses, and that it is able to distinguish between stars and BHs of high
mass, when the finite size of the source is taken into account. A significant
presence of massive BHs would produce frequent high flux magnifications (except
for unrealistically large sources) which have been very rarely observed. On the
contrary, a typical stellar population would induce flux magnifications
consistent with the observations. This result excludes PBHs (or any type of
compact object) in the mass range determined by LIGO/Virgo as the main dark
matter constituents in the lens galaxies.
