We present a comprehensive study of phase fluctuation effects in ultrathin Pb films using combined scanning tunneling microscopy and two-coil mutual inductance measurements. In atomically thin films (two to four monolayers), we observe a pronounced separation between the Cooper pair formation temperature ($T_{\mathrm{\Delta }}$) and the macroscopic superconducting transition temperature ($T_{\mathrm{c}}$), signaling the emergence of incoherent Cooper pairs and a pseudogap regime above $T_{\mathrm{c}}$. The superfluid stiffness $J_{\mathrm{n}}$($T$) exhibits distinct scaling behaviors: a Berezinskii-Kosterlitz-Thouless (BKT)-like transition with renormalized vortex dynamics in 2-ML films, and a crossover from quantum to classical phase fluctuations in 4-ML films. Thicker films (7 to 11 ML) adhere to conventional Bardeen-Cooper-Schrieffer theory, demonstrating negligible phase fluctuations despite their subcoherence-length thickness. Our results establish the dominance of phase fluctuations in low-dimensional superconductors and provide a clean experimental platform to explore BKT and incoherent pairing physics in highly crystalline systems.

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• BKT transition
• Superconducting fluctuations
• Superconductivity
• Thin films