  It is a fundamental property of the Chow groups of algebraic schemes that
they are contra-functorial with respect to flat morphisms between schemes.
While the pullback homomorphism is easy to define at the level of algebraic
cycles, the crucial step is to show that the pullback of cycles preserves
rational equivalence, so that it descends to the Chow groups. The purpose of
this note is to give a natural sheaf theoretic proof of the preservation of
rational equivalence under flat pullback on cycles.
