In theory, Account Based Marketing (ABM) is a narrowing of the funnel.
In exchange for concentrating marketing resources on a limited number of accounts, you trade a higher Cost per Lead (CPL) for the promise of better conversion rates, lower churn, higher Average Customer Value, etc.
However, in practice, ABM does not mean you have to limit your reach at the top of the funnel. Account based tactics can be used to compliment just about any approach to demand generation.
In this whiteboard video, Lena Shaw, Director of Marketing and Growth at LeadGenius, demonstrates how outbound email campaigns can be used by marketing teams to accelerate lead velocity at all stages of the funnel.