  The absence of self averaging in mesoscopic systems is a consequence of
long-range intensity correlation. Microwave measurements suggest and
diagrammatic calculations confirm that the correlation function of the
normalized intensity with displacement of the source and detector, $\Delta R$
and $\Delta r$, respectively, can be expressed as the sum of three terms, with
distinctive spatial dependences. Each term involves only the sum or the product
of the square of the field correlation function, $F \equiv F_{E}^2$. The
leading-order term is the product, the next term is proportional to the sum.
The third term is proportional to $[F(\Delta R)F(\Delta r) + [F(\Delta
R)+F(\Delta r)] + 1]$.
