  It is shown that the idea of ``minimal'' coupling to gauge fields can be
conveniently implemented in the proper time formalism by identifying the
equivalent of a ``covariant derivative''. This captures some of the geometric
notion of the gauge field as a connection. The proper time equation is also
generalized so that the gauge invariances associated with higher spin massive
modes can be made manifest, at the free level, using loop variables. Some
explicit examples are worked out illustrating these ideas.

High Energy Physics - Theory