  Coherence lengths of one particle states described by quantum wave functions
are studied. We show that one particle states in various situations are not
described by simple plane waves but are described by wave packets that are
superpositions of plane waves. Wave packet is an approximate eigenstate of the
free Hamiltonian and has a finite spatial size which we call the coherence
length. The coherence lengths in the coordinate space and in the momentum space
are studied in the present paper. We investigate several mechanisms of forming
wave packets, stabilities of wave packets, and transformations of wave packets.
