First of all, et al. should be in italics. That said, the common way to refer to a publication like that would be:

Smith and coworkers'...

Et al. means and others, it is an abbreviation of the latin et alii. If you really really wanted to use it in the possessive, you would write

Smith et al's

but don't do that, it is ugly and unclear. Paraphrase, use and coworkers or similar constructs.

First of all, et al. should be in italics. That said, the common way to refer to a publication like that would be:

Smith and coworkers'...

or

Smith and colleagues'

Et al. means and others, it is an abbreviation of the latin et alii. If you really really wanted to use it in the possessive, you would write

Smith et al's

but don't do that, it is ugly and unclear. Paraphrase, use and coworkers or similar constructs.

First of all, et al. should be in italics. That said, the common way to refer to a publication like that would be:

Smith and coworker's...

Et al. means and others, it is an abbreviation of the latin et alii. If you really really wanted to use it in the possessive, you would write

Smith et al's

but don't do that, it is ugly and unclear. Paraphrase, use and coworkers or similar constructs.

First of all, et al. should be in italics. That said, the common way to refer to a publication like that would be:

Smith and coworkers'...

Et al. means and others, it is an abbreviation of the latin et alii. If you really really wanted to use it in the possessive, you would write

Smith et al's

but don't do that, it is ugly and unclear. Paraphrase, use and coworkers or similar constructs.