  In order to investigate when neutrinos cease to oscillate in the framework of
quantum field theory, we have reexamined the wave packet treatment of neutrino
oscillations by taking different sizes of the wave packets of the particles
involved in the production and detection processes. The treatment is shown to
be considerably simplified by using the Grimus-Stockinger theorem which enables
us to carry out the integration over the momentum of the propagating neutrino.
Our new results confirm the recent observation by Kiers, Nussinov and Weiss
that a precise measurement of the energies of the particles involved in the
detection process would increase the coherence length. We also present a
precise definition of the coherence length beyond which neutrinos cease to
oscillate.
