How to use Google Cross-Client Identity on Android to authenticate Web API 2 within the same project?

I've been researching Authentication and Authorization using ASP.NET Web API 2 for 2-3 weeks, and after a lot of research and reading, I can agree on the underlying architecture for my project.

My project currently consists of:

• ASP.NET Web API Server 2 (OWIN / Katana)
• Android app

I am referencing "Multiple Client Authentication" to create access_token and refresh_token for Web API Project 2 ; where access_token is generated in android app for web API using GoogleAuthUtil.getToken ()

The access token (transporter) does not work in the web API and is significantly shorter than the one generated on the web API side using OWIN middleware.

I think I am exactly following all the steps correctly, unless I am doing something fundamentally wrong.