External audio drifts out of sync with video taken from iphone.

Hello, I'm trying to sync audio taken with a zoom H4n with video taken by an iphone 6s. When I import the external audio (.wav, 44.1Hz) into iMovie, it slowly goes out of sync with the iphone video (.mov). Comparing the external audio to the iphone video audio, the slow drift out of sync is obvious. However, when I import the audio from the video as well as the external audio into audacity, they line up perfectly. I believe both audio tracks are 44.1Hz, and the video seems to be using a VBR. I tried converting the external audio to 48 Hz, but that didn't seem to make a difference, as it still drifted.

Is there any way to make them sync up properly? I'm using iMovie 10.0.8 on Yosemite (10.10.5). I appreciate your help in advance!