  A model is proposed to describe a transition from a Schwarzschild black hole
of mass $M_{0}$ to a Schwarzschild black hole of mass $M_{1}$ $\leq M_{0}$. The
basic equations are derived from the non-vacuum Einstein field equations taking
a source representing a null scalar field with a nonvanishing trace anomaly. It
is shown that the nonvanishing trace anomaly of the scalar field prevents a
complete evaporation.
