  At this summer school, I have tried to describe a general approach to quantum
transport problems based on the Non-equilibrium Green Function (NEGF) method .
I will not repeat this treatment here. Instead I will use this article to draw
attention to a core issue in quantum transport and how it is modeled within the
NEGF method. Specifically, I will discuss what constitutes a dephasing process
and how the standard descriptions may need to be extended to include what I
will call hot spin effects. These involve a number of subtle issues that tend
to get hidden in a purely formal description. So I have chosen instead to
illustrate the concepts using a concrete device, which is specially engineered
to enhance its sensitivity to spin dephasing and hot spin effects.
