iFixIt says DDR3, but it looks like they have only done a teardown of the M3 model. So I am not sure if MS used different memory for the i5 and i7 models:
Intel Core m3-6Y30:
RAM: 4GB DDR3L 1600 MHz
Storage: 128 GB NVMe SSD
Intel Core i5-6300U:
RAM: 4 GB/8 GB/16 GB DDR3L 1600 MHz
Storage:
With 4 GB of RAM: 128 GB NVMe SSD
With 8 GB/16GB of RAM :256 GB/512 GB NVMe SSD
Intel Core i7-6650U:
RAM: 8 GB/16 GB DDRL3 1600 MHz
Storage:
With 8 GB of RAM: 256 GB NVMe SSD
With 16 GB of RAM: 256 GB/ 512 GB/ 1 TB NVMe SSD