Heat transfer at the nanoscale is fundamentally different from that at the macroscale and is determined by the distribution of the mean free paths of energy carriers in a material,the length scales of the heat sources,and the distance over which the heat is transported.Past work has shown that Fouriers law for heat conduction valid at the bulk and continuum level dramatically over-predicts the rate of heat dissipation from heat sources with dimensions smaller than the mean free path of the dominant heat-carrying phonons.In this work,we uncover a new regime of nanoscale heat conduction that dominates when the separation between nanoscale heat sources is small compared with the dominant phonon mean free paths.Surprisingly,the interplay between neighboring heat sources can facilitate efficient diffusive-like heat dissipation.This finding suggests that the thermal management problem in nanoscale integrated circuits might not be as serious as projected.Finally,we demonstrate a unique and new capability to extract mean free path distributions in materials,allowing the very first experimental validation of differential conductivity predictions from first-principles calculations.