  In this paper we consider soft high-energy parton-parton scattering processes
in gauge theories, i.e., elastic scattering processes involving partons at very
high squared energies in the center of mass and small squared transferred
momentum. By a direct resummation of perturbation theory in the limit we are
considering, we derive expressions for the truncated-connected quark
(antiquark) propagator in an external gluon field, as well as for the residue
at the pole of the full unrenormalized propagator, both for scalar and fermion
gauge theories. These are the basic ingredients to derive high-energy
parton-parton scattering amplitudes, using the LSZ reduction formulae and a
functional integral approach. The above procedure is also extended to include
the case in which at least one of the partons is a gluon. The meaning and the
validity of the results are discussed.
