  The transport properties of the quark-gluon plasma is important both from the
point of view of the astrophysical systems and the heavy ion collisions.
Therefore, accurate calculations of the elastic scattering cross-sections of
$qq' \to qq'$ and $ q\qbar' \to q\qbar'$ processes are required. At the
tree-diagram level exchange of massless gluons leads to divergent cross
sections for such processes. An effective way to remove the infrared divergence
is to introduce a Debye mass as an infrared regulator into the gluon propagator
for the thermal gluons. In this work we find that at the two-gluon exchange
level such a technique fails to remove the divergences. In a chiral symmetry
broken phase the mass could be introduced following the standard procedure
which cures the divergence. But in a chirally symmetric phase chiral invariant
mass of the fermions needs to be considered.
