I had it backwards. SQL Server needed to implicitly convert each of the varchar values in your table to nvarchar prior to comparing it to your constant value N'123456789', thus requiring it to scan the index rather than just doing a seek.

If you right click on the index scan in your actual execution plan and click on properties, then scroll down the properties page to the bottom and look at the Predicate, you will see the implicit conversion that SQL Server needed to accomplish for the comparision.