W49B is the youngest supernova remnant (SNR) to date that exhibits recombining plasma. The two prevailing theories of this overionization are rapid cooling via adiabatic expansion or through thermal conduction with an adjacent cooler medium. To constrain the origin of the recombining plasma in W49B, we perform a spatially resolved spectroscopic study of deep XMM-Newton data across 46 regions. We adopt a three-component model (with one interstellar medium and two ejecta components), and we find that recombining plasma is present throughout the entire SNR, with increasing overionization from east to west. The latter result is consistent with previous studies, and we attribute the overionization in the west to adiabatic expansion. However, our findings depart from these prior works, as we find evidence of overionization in the east as well. As the SNR is interacting with molecular material there, we investigate the plausibility of thermal conduction as the origin of the rapid cooling. Based on estimated timescales, we show that both large- and small-scale thermal conduction can explain the observed overionization of the hotter ejecta. However, overionization of the cooler ejecta is only possible through small-scale thermal conduction resulting in evaporation of embedded, dense clouds with sizes less than or similar to 1 pc.