In this study, we present a robust high aspect ratio confined PCM cell which utilizes a dense nano-crystalline-as-deposited phase change material. The small pore structures were filled utilizing an in-situ metallic liner plus nano-crystalline ALD GST process. The tuned process for depositing and integrating the phase change material yields a void-free dense confined PCM inside the pore. This confined PCM with a metallic liner is found to be immune to classic endurance failure mechanisms. So, the highly reliable confined PCM yields a new record endurance (2x1012 cycles) and a fast programming speed of 80ns with 10x switching. In addition, we thoroughly investigate the excellent endurance of the novel confined PCM with a thin metallic liner. Experimental results confirm that both the proper metallic liner and the confined pore cell structure are required for a reliability advantage. This confined PCM cell has high potential of being incorporated as a M-type storage class memory with advanced memory technology node due to its outstanding endurance and fast set speed.