  After decades of explorations, suffering from low critical temperature and
subtle nature, whether a metallic ground state exists in a two-dimensional
system beyond Anderson localization is still a mystery. Supremely, phase
coherence could be the key that unlocks its intriguing nature. This work
reveals how quantum phase coherence evolves across bosonic
superconductor-metal-insulator transitions via magneto-conductance quantum
oscillations in high-Tc superconducting films. A robust intervening anomalous
metallic state characterized by both resistance and oscillation amplitude
saturations in the low temperature regime is detected. By contrast, with
decreasing temperature the oscillation amplitude monotonically grows on the
superconducting side, but decreases at low temperatures on the insulating side.
It suggests that the saturation of phase coherence plays a prominent role in
the formation of this anomalous metallic state.
