  It is well known that neutrino oscillations may damp due to decoherence
caused by the separation of mass eigenstate wave packets or by a baseline
uncertainty of order the oscillation wave length. In this note we show that if
the particles created together with the neutrino are not measured and do not
interact with the environment, then the first source of decoherence is not
present. This demonstration uses the saddle point approximation and also
assumes that the experiment lasts longer than a certain threshold. We
independently derive this result using the external wave packet model and also
using a model in which the fields responsible for neutrino production and
detection are treated dynamically. Intuitively this result is a consequence of
the fact that the neutrino emission time does not affect the final state and so
amplitudes corresponding to distinct emission times must be added coherently.
This fact also implies that oscillations resulting from mass eigenstates which
are detected simultaneously arise from neutrinos which were not created
simultaneously but are nonetheless coherent, realizing the neutrino oscillation
paradigm of Kobach, Manohar and McGreevy.
