Single-cell sequencing dataset has too many barcodes
The BAM file you downloaded has all of the detected 10X barcodes instead of those that represent cells.  In a given 10X experiment the number of input barcodes vastly outnumbers the input cell count.  After the reaction some of barcoded beads are invariably sequenced because they are encapsulated with ambient RNA, dead cells etc. After processing the BAM file Cell Ranger infers which cells likely represent cells and generates gene expression matrices from these files.  

You would need to download one of the output files that contains the list of passed barcodes such as the Gene / cell matrix (filtered) and then use the list of passed barocode sequences to filter your BAM file. Alternatively you could approximately select the same cells by selecting the top 2,000 barcodes with the highest numbers of reads.