  We use angle-resolved photoemission to unravel the quasiparticle decoherence
process in the high-$T_c$ cuprates. The coherent band is highly renormalized,
and the incoherent part manifests itself as a nearly vertical ``dive'' in the
$E$-$k$ intensity plot that approaches the bare band bottom. We find that the
coherence-incoherence crossover energies in the hole- and electron-doped
cuprates are quite different, but scale to their corresponding bare bandwidth.
This rules out antiferromagnetic fluctuations as the main source for
decoherence. We also observe the coherent band bottom at the zone center, whose
intensity is strongly suppressed by the decoherence process. Consequently, the
coherent band dispersion for both hole- and electron-doped cuprates is
obtained, and is qualitatively consistent with the framework of Gutzwiller
projection.
