  We consider an electron magnetically interacting with a spin-1/2 impurity,
embedded in an external environment whose noisy term acts only on the
impurity's spin, and we find expressions for the electron transmission and
reflection probabilities in terms of the phenomenological noise parameters.
Moreover, we give a simple example of the necessity of complete positivity for
physical consistency, showing that a positive but not completely positive
dissipative map can lead to negative transmission probabilities.
